Recently, wireless communication systems such as an HSPA (High Speed Packet Access) and LTE (Long Term Evolution) have been developed. In such HSPA and LTE, to achieve higher efficiency and higher reliability data transmission, a technique such as an AMC (Adaptive Modulation and Coding scheme) has been employed.
In the AMC, depending on the quality of a radio channel, an MCS (Modulation and Coding Scheme) of a data signal is performed. Specifically, in the MCS, an appropriate modulation scheme, coding rate, or a combination thereof is selected (controlled). By doing this, while maintaining a predetermined level of the received quality, the MCS having a higher efficiency may be achieved and accordingly the transmission efficiency of data may be improved.
Next, a configuration of a wireless communication system employing the AMC in downlink in the related art is described with reference to FIGS. 1 and 2.
FIG. 1 illustrates an example configuration of a base station in the related art. In FIG. 1, an RF receiver 11 receives a signal fed back from a mobile station, converts the received radio-frequency signal to a baseband signal, performs quadrature demodulation and A/D conversion, and outputs the A/D converted signal to a control signal decoder 12.
The control signal decoder 12 decodes a control signal and extracts a CQI (Channel Quality Indicator) value from the decoded signal. The CQI value indicates a quality of the radio channel by using four-bit data. Herein, the CQI value is calculated based on a radio received quality (e.g., an SINR (Signal-to-Interference and Noise power Ratio)) measured by the mobile station. Further, in this case, the CQI value is calculated so that a BLER (Block Error Rate) is 10% when a data signal having a transmission format corresponding to the CQI value is received.
For example, in the LTE, the modulation, the coding rate, and the number of information bits transmitted in one modulation symbol (Efficiency) correspond to 16 levels (i.e., 1 to 15 levels) of a CQI index. Further, the greater the CQI index is, the greater the quality of a radio channel becomes. The control signal decoder 12 outputs the CQI index to an MCS selector 13. The CQI index is expressed by using four CQI bits which are extracted from the decoded control signal.
The MCS selector 13 selects the MCS of the data signal (i.e., a combination of the modulation and the coding rate) based on the CQI index. Generally, the greater the CQI index is, the higher the efficiency of the selected MCS becomes.
A data signal generator 14 performs error correction coding on the data signal (information bits) so that the coding rate is equal to a value indicated in the MCS. Further, the data signal generator 14 performs data modulation in accordance with the modulation indicated in the MCS. A control signal generator 15 generates a control signal by performing coding, data modulation and the like on the control information including the MCS.
A pilot signal generator 16 generates a pilot signal necessary for decoding the data signal and the control signal and measuring the CQI in the mobile station. A channel multiplexer 17 multiplexes the data signal, the control signal, and the pilot signal and generates a signal in a predetermined radio access scheme (e.g., the OFDMA). An RF transmitter 18 performs D/A conversion and quadrature modulation, converts the baseband signal into the radio-frequency signal, and transmits the radio-frequency signal.
FIG. 2 illustrates an example configuration of a mobile station in the related art. In FIG. 2, an RF receiver 21 receives a signal transmitted from the base station, converts the received radio-frequency signal to a baseband signal, performs quadrature demodulation and A/D conversion, and outputs the A/D converted signal to a channel separation section 22.
The channel separation section 22 separates the input signal into the data signal, the control signal, and the pilot signal by performing receiving processes (e.g., in case of OFDMA, an FFT timing detecting process, a GI removal process, and an FFT process) on the signal in the predetermined radio access scheme (e.g., OFDMA).
An channel estimator 23 estimates a CSI (Channel State Information) of the radio channel by calculating a correlation value between the pilot signal received from the channel separation section 22 and a known pilot signal. The CSI is expressed in a complex number.
A CQI calculator 24 calculates the four-bit CQI index based on the radio received quality (e.g., an SINR) estimated using the CSI. Specifically, as described above, the CQI is calculated so that a BLER (Block Error Rate) is 10% when a data signal having a transmission format corresponding to the CQI is received.
A control signal decoder 25 performs channel compensation on the received control signal from the channel separation section 22 based on the CSI from the CQI calculation section 24. Further, the control decoder 25 restores control information (including the MCS) by performing data demodulation and error correction decoding.
A channel compensator 26 performs channel compensation on the received data signal from the channel separation section 22 based on the CSI from the channel estimator 23. A data signal decoder 27 decodes the data based on the modulation indicated in the MCS from the control signal decoder 25, and restores and outputs information bit data by performing the error correction decoding on the data decoded using the coding rate indicated in the MCS.
A control signal generator 28 generates a control signal by performing coding, data modulation and the like on control information including the four-bit CQI index from the CQI calculator 24. An RF transmitter 29 performs D/A conversion and quadrature modulation, converts the baseband signal into the radio-frequency signal, and transmits the radio-frequency signal to the base station.
Further, there is a proposed technique in which, in LTE, the channel state information and the acknowledgement for the downlink data signal are multiplexed, the multiplexed control signal is channel coded, and the channel-coded signal is transmitted using the uplink control channel (PUCCH) (see, for example, Japanese Laid-open Patent Publication No. 2008-236432).
Whether the AMC can be precisely operated generally depends on a quality of the fed-back CQI index. Especially, when a bit error occurs and the base station cannot detect the bit error in the feedback process, the CQI index greatly different from the CQI index calculated in the mobile station may be decoded. In this case, the MCS greatly different from the optimum MCS for the actual radio channel may be selected. As a result, the data throughput may be greatly degraded.
For example, in an LTE system, the UCI (Uplink Control Information) including the CQI index is transmitted using the physical uplink shared channel (PUSCH) or the physical uplink control channel (PUCCH) depending on the conditions.
Specifically, when there exist the data to be transmitted, the data and the UCI are time-domain multiplexed and the multiplexed data with error detection code bits (CRC) are transmitted by using the PUSCH. On the other hand, however, when there are no data to be transmitted, the UCI without additional CRC bits is transmitted by using the PUCCH.
As described above, when the PUCCH is used, no CRC bits are added. Therefore, when a bit error remains after decoding, it may be difficult to detect the bit error. As a result, as described above, the MCS greatly different from the optimum MCS for the actual radio channel may be selected, and the data throughput may be greatly degraded.